An amplifier is an electronic device that can increase the power of a signal by taking energy from a power supply and controlling the output to match the input signal shape, while increasing the amplitude. Amplifiers are used in multiple aspects of electronic circuits and particularly in analog circuits. A specific type of amplifier is the operational amplifier (op-amp). Op-amps are used in consumer, industrial, and scientific devices, for example. Op-amps may be packaged as components, or used as elements of more complex integrated circuits (ICs). Op-amps may be implemented using numerous circuit fabrication techniques and are often fabricated as a CMOS IC. Op-amps are generally implemented using one or more stages with or without a compensating network. The numerous configurations and implementations of various op-amps are often related to the particular implementation and intended usage.
One of the main constraints for an uncompensated two-stage CMOS op-amp is an inherently small frequency spacing between a first and a second pole, which causes insufficient phase margin and leads to instability in closed-loop conditions. Different frequency compensation techniques are used in order to overcome this limit. The most popular technique is Miller compensation, which achieves pole splitting by using a compensation capacitor placed around a second stage of the op-amp. For Miller compensation, the dominant pole may be shifted to a lower frequency due to the increased effective capacitance at the output node of the first stage, whereas the non-dominant pole is pushed to a higher frequency as a result of the reduced output impedance of the second stage at sufficiently high frequencies. Miller compensation does generally produce a right half-plane (RHP) zero, which is caused by a feed-forward path introduced by the compensation capacitor. The frequency of this zero may be on the same order of magnitude of the unity-gain frequency of the op-amp since the transconductances of the two stages are generally similar in CMOS technology, which significantly degrades the phase margin of the op-amp and reduces stability.
There are different techniques used with Miller compensation for addressing the RHP zero. The different techniques include nulling-resistor compensation (NRC), voltage-buffer compensation (VBC), and current-buffer compensation (CBC). Nulling-resistor compensation uses a resistor connected in series with the compensation capacitor, which moves the zero to a new frequency. If a proper value of the resistor is chosen, the zero approaches infinity and, hence, reduces instability problems. Both current-buffer and voltage-buffer compensations (CBC and VBC), rather than relocating the RHP zero, prevent the formation of the RHP zero by cutting the feed-forward path originated by the compensation capacitor.
For example, voltage-buffer compensation includes placing a unity-gain buffer between the output of the second stage and the right plate of the compensation capacitor, so that the signal voltage across the compensation capacitor is the same as in the case of standard Miller compensation. As another example, current-buffer compensation includes placing a unity-gain current-buffer between the left plate of the compensation capacitor, as depicted in a conventional circuit schematic, and the output of the first stage, so that the signal current injected into the latter node is the same as in standard Miller compensation. In some cases, current-buffer compensation allows obtaining a larger gain-bandwidth product; however, current-buffer compensation design is not as straightforward as voltage-buffer compensation due to the non-zero input impedance of the current-buffer, which can cause the formation of complex and conjugates poles that may lead to instability.